EP3919165B1

EP3919165B1 - Method for forming a catalyst article

Info

Publication number: EP3919165B1
Application number: EP20177967.5A
Authority: EP
Inventors: Nikki Jane Amos-Reichert; Daniel Robert AVIS; Jürgen Bauer; Yannick BIDAL; Guy Richard Chandler; Alexander Michael GREEN; Neil Greenham; Matthew Harris; Sofia LOPEZ-OROZCO; Jörg Werner MÜNCH; Paul Richard Phillips; Irene Piras
Original assignee: Johnson Matthey Catalysts Germany GmbH; Johnson Matthey PLC
Current assignee: Johnson Matthey Catalysts Germany GmbH; Johnson Matthey PLC
Priority date: 2020-06-03
Filing date: 2020-06-03
Publication date: 2025-08-13
Anticipated expiration: 2040-06-03
Also published as: CN115515713A; EP3919165A1; EP4609948A2; JP7785685B2; EP4609948A3; BR112022022267A2; US20210379576A1; CN120644235A; JP2023527956A; WO2021245375A1

Description

Field of the Invention

The present invention relates to a method for forming a catalyst article. In particular, the present invention relates to a method for forming a catalyst article suitable for use in the selective catalytic reduction of nitrogen oxides (NOx) in an exhaust gas.

Background of the Invention

Large numbers of catalytic converters used for the treatment of emissions from mobile and stationary sources are manufactured each year. Catalytic converters for use in motor-vehicles typically comprise an extruded ceramic monolith that is provided with channels for the through-flow of exhaust gases. The channels of the monolith may be coated with a catalytically active material. Alternatively, the extruded monolith itself is formed of a catalytically active material (referred to as an "all-active extrudate" or "extruded catalyst").
In the production of a coated catalyst, a composition known as a "washcoat" is applied to a substrate (e.g. a ceramic monolith). A washcoat typically comprises a liquid and a catalytically active material. The washcoat may take the form of a solution, slurry or suspension of catalytic material in a solvent. Once coated onto the substrate, the washcoat typically undergoes a calcination step, to remove solvent and to fix the catalytically active material to the substrate.
Substrates for use in catalytic converters generally comprise a unitary structure in the form of a honeycomb having uniform-sized and parallel channels extending from a first end to a second end thereof. Generally, the channels are open at both the first and second ends - a so-called "flow through" configuration. Alternatively, channels at a first, upstream end can be plugged, e.g. with a suitable ceramic cement, and channels not plugged at the first, upstream end can also be plugged at a second, downstream end to form a so-called wall-flow filter.
The selective catalytic reduction of nitrogen oxides (NO_x) by ammonia (NH₃-SCR) is considered to be the most practical and efficient technology for the abatement of NO_x from exhaust gases emitted from stationary sources and mobile engines, principally diesel engines for vehicles such as automobiles, trucks, locomotives and ships.
Known SCR (selective catalytic reduction) catalysts include molecular sieves. Useful molecular sieves include crystalline or quasi-crystalline materials which can be, for example aluminosilicates (zeolites) or silicoaluminophosphates (SAPOs). Such molecular sieves are constructed of repeating SiO₄, AlO₄, and optionally PO₄ tetrahedral units linked together, for example in rings, to form frameworks having regular intra-crystalline cavities and channels of molecular dimensions. The specific arrangement of tetrahedral units (ring members) gives rise to the molecular sieves framework, and by convention, each unique framework is assigned a unique three-letter code (e.g., "CHA") by the International Zeolite Association (IZA). Examples of molecular sieve frameworks that are known SCR catalysts include Framework Type Codes CHA (chabazite), BEA (beta), MOR (mordenite), AEI, MFI and LTA.
Molecular sieves (e.g. zeolites) may also be categorised by pore size, e.g. a maximum number of tetrahedral atoms present in a molecular sieve's framework. As defined herein, a "small pore" molecular sieve, such as CHA, contains a maximum ring size of eight tetrahedral atoms, whereas a "medium pore" molecular sieve, e.g. MFI, contains a maximum ring size of ten tetrahedral atoms; and a "large pore" molecular sieve, such as BEA, contains a maximum ring size of twelve tetrahedral atoms. Small and medium pore molecular sieves, especially small pore molecular sieves, are preferred for use in SCR catalysts, since they may, for example, provide improved SCR performance and/or improved hydrocarbon tolerance.
Molecular sieve catalysts, may be metal-promoted. Examples of metal-promoted molecular sieve catalysts include iron-, copper- and palladium-promoted molecular sieves, where the metal may be loaded into the molecular sieve. In a metal-loaded molecular sieve, the loaded metal is a type of "extra-framework metal", that is, a metal that resides within the molecular sieve and/or on at least a portion of the molecular sieve surface and does not include atoms constituting the framework of the molecular sieve. Iron- and copper-loaded zeolites, for example, are known to promote SCR reactions.
Several methods have been mentioned in the literature for preparing metal-loaded molecular sieves, in particular metal-loaded zeolites. The direct synthesis of metal-loaded zeolites is a complicated process and depends on the synthesis conditions (see M. Moliner, ISRN Materials Science, 2012, Article ID 789525). An alternative is to use a commercial zeolite support and to subsequently add metal by post-synthesis treatment of the zeolite, for example, by wet impregnation, wet ion exchange or solid-state ion exchange.
Known wet ion-exchange methods for the addition of metal to molecular sieves (e.g. zeolites) typically employ soluble metal salts, such as metal acetates, metal sulphates or metal chlorides as the active metal precursor, wherein the active metal precursor is reacted with the molecular sieve in aqueous solution. In order to accelerate ion-exchange, such processes typically require a heating step, wherein the mixture may be heated to a temperature in the range 70 to 80°C for up to several hours. Further, additional processing steps (e.g. filtering, evaporation, spray-drying etc) may be required before the resulting metal-loaded molecular sieve may be employed in a washcoat composition for the formation of a catalyst article. Further still, it has been found that where certain metal-acetates (e.g. copper actetate) are employed to prepare metal-loaded molecular sieves (e.g. metal-loaded zeolites) for use as SCR catalysts, any residual metal acetate remaining after calcination may have a poisoning effect on ammonia slip catalysts (ASCs) which are used downstream of or proximal to the SCR catalyst.
JP H11 147041 A relates to a catalyst for exhaust gas-cleaning that reduces nitrogen oxide (NOx) contained in exhaust gas discharged from a diesel engine and cleans the exhaust gas. The catalyst comprises a homogenized mixture obtained by mixing preferably the matrix material powder and a copper oxide with a binder to a solidified state and copper oxide particles as a catalytic active specifies are dispersed in the matrix or thereon. Alternatively, the catalyst is formed by dispersing on/in a matrix made up of zeolite and/or a metallic ferrite at least one kind of precious metal particle selected from platinum, rhodium, iridium and silver, and copper oxide particles in the matrix or thereon.
US 2015/252708 A1 relates to a zoned catalysed substrate monolith comprising a first zone and a second zone that are arranged axially in series. The first zone comprises a platinum group metal loaded on a support and a first base metal oxide or a first base metal loaded on an inorganic oxide. The first base metal oxide is iron oxide, manganese oxide, copper oxide, zinc oxide, nickel oxide, or mixtures thereof. The first base metal is iron, manganese, copper, zinc, nickel, or mixtures thereof. The second zone comprises copper or iron loaded on a zeolite and a second base metal oxide or a second base metal loaded on an inorganic oxide. The second base metal oxide is iron oxide, manganese oxide, copper oxide, zinc oxide, nickel oxide, or mixtures thereof. The second base metal is iron, manganese, copper, zinc, nickel, or mixtures thereof. The second base metal is different from the first base metal.
US 2015/246345 A1 relates to SCR-active molecular sieve based-catalysts produced by combining a molecular sieve with at least one ionic iron species and at least one organic compound to form a mixture, then calcining the mixture to remove the at least one organic compound.
US 2020/086303 A1 relates to a catalyst that includes LTA zeolite including copper ions, wherein a Si/Al ratio of the LTA zeolite is 2 to 50. The catalyst is coated on a honeycomb carrier or a filter.
US 2017/259248 A1 relates to a coating suspension containing at least one platinum group metal on a support material, as well as manganese(II) carbonate, and to a method for coating a catalyst support substrate.
EP 3015167 A1 relates to an ammonia oxidation catalyst for converting nitrogen oxides generated from a mobile source or fixed source into harmless nitrogen using ammonia as a reductant and preventing the formation of nitrogen oxides due to the oxidation of ammonia. The ammonia oxidation catalyst includes selective catalytic reductive zeolite sequentially impregnated with platinum and copper.
WO 2020/089043 A1 relates to an exhaust treatment system comprising a Diesel Oxidation Catalyst (DOC), a Catalyzed Soot Filter (CSF), a reductant injector, an AEI zeolite based Selective Catalyzed Reduction (SCR) catalyst and an Ammonia Oxidation Catalyst (AMOX) downstream to the AEI zeolite based SCR catalyst.
US 2019/322537 A1 relates to the preparation of a SCR catalyst article comprising zeolite Cu-CHA by the one-pot slurry ion exchange method (ISIE). In the examples, a slurry is formed by mixing in water zeolite CHA in the H-form, copper oxide, Zr acetate binder, and an additive to form a slurry, which is then milled and applied to the inner walls of a honeycomb carrier, dried and calcined.
The present invention provides an improved process for the preparation of washcoated catalyst articles which employ a metal-loaded crystalline molecular sieve as a catalytically active material.
According to the present invention there is provided a method for forming a catalyst article comprising:

(a) forming a slurry by mixing together at least the following components:
1. (i) a crystalline molecular sieve in an H⁺ or NH₄ ⁺ form, wherein the crystalline molecular sieve is a small pore zeolite having a Framework Type that is CHA;
2. (ii) a water-insoluble active metal precursor, wherein the water-insoluble active metal precursor is copper (II) carbonate;
3. (iii) an aqueous solvent, wherein the aqueous solvent is water;
wherein the slurry has a solids content of up to 50 weight% and wherein step (a) is carried out at a temperature in the range 10 to 35°C;
(b) coating a substrate with the slurry formed in step (a); and
(c) calcining the coated substrate formed in step (b) to form a catalyst layer on the substrate.

In the method the heat employed to calcine the coated substrate may be exploited to promote metal-loading onto the molecular sieve. Thus, the requirement for any heating steps during wet ion-exchange or impregnation processes and the requirement for expensive, high-temperature-resistant equipment may be avoided. Further, long reaction times typical in wet ion-exchange or impregnation processes and/or energy and labour-intensive processes such as spray-drying may be avoided. Consequently, the method according to the invention may be more energy efficient and economical.
Furthermore, the slurry prepared in step (a) of the method according to the invention may be employed directly as a washcoat composition without the need for any further processing steps.
Further still, the use of insoluble metal species, namely metal carbonates, as active metal precursors may result in the generation of fewer hazardous species during calcination compared to when metal acetates are used as active metal precursors. Thus, the use of insoluble active metal precursors may provide health and safety benefits.
Additionally, it has been found that catalysts prepared via the process according to the present invention may provide at least comparable SCR activity to catalysts comprising metal-loaded molecular sieves (e.g. metal-loaded zeolites) which were prepared via wet ion exchange or impregnation. Moreover, it has been found that poisoning of associated ammonia slip catalysts may be mitigated compared to catalysts comprising metal-loaded molecular sieves which have been prepared using metal acetates as the active metal precursor.
Not according to the claimed invention, there is provided a catalyst article obtained or obtainable according to the method of the invention.
Not according to the claimed invention, there is provided an exhaust system comprising: a source of nitrogenous reductant and an injector for injecting a nitrogenous reductant into a flowing exhaust gas, wherein the injector is disposed upstream from a catalyst article as defined above.

Brief Description of the Drawings

Figure 1 is a graph showing NO_x conversion and N₂O selectivity achieved by a catalyst article prepared according to the method of the invention compared with a catalyst article prepared via a prior art method.
Figure 2 is a graph showing NOx conversion and N₂O selectivity achieved by catalysts prepared according to the method of the invention.

Detailed Description

The present disclosure will now be described further. In the following passages different aspects/embodiments of the disclosure are defined in more detail. Each aspect/embodiment so defined may be combined with any other aspect/embodiment or aspects/embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.
Further, the term "comprising" as used herein can be exchanged for the definitions "consisting essentially of" or "consisting of". The term "comprising" is intended to mean that the named elements are essential, but other elements may be added and still form a construct within the scope of the claim. The term "consisting essentially of" limits the scope of a claim to the specified materials or steps and those that do not materially affect the basic and novel characteristic(s) of the claimed invention. The term "consisting of" closes the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith.
The crystalline molecular sieve is typically composed of aluminium, silicon, and/or phosphorus. The crystalline molecular sieve generally has a three-dimensional arrangement (e.g. framework) of repeating SiO₄, AlO₄, and optionally PO₄ tetrahedral units that are joined by the sharing of oxygen atoms.
The term "H⁺-form" in relation to a molecular sieve refers to a molecular sieve having an anionic framework wherein the charge of the framework is counterbalanced by protons (i.e. H⁺ cations).
The term NH₄ ⁺ form in relation to a molecular sieve refers to a molecular sieve having an anionic framework wherein the charge of the framework is counterbalanced by ammonium cations (NH₄ ⁺ cations).
The molecular sieve is a zeolite.
The molecular sieve is a small pore molecular sieve (i.e. having a maximum ring size of eight tetrahedral atoms).
The small pore crystalline molecular sieve has a Framework Type that is CHA.
The zeolite may have a silica-to-alumina ratio (SAR) of 5 to 200, preferably 5 to 100, more preferably 10 to 80. For example, the zeolite may have a silica-to-alumina ratio (SAR) of 5 to 30.
The crystalline molecular sieve is preferably a powdered crystalline molecular sieve (i.e. in particulate form), wherein the particles comprise individual crystals, agglomerations of crystals or a combination of both. The crystalline molecular sieve may have a mean crystal size, as measured by scanning electron microscopy (SEM), of ≥ 0.5 µm, preferably between about 0.5 and about 15 µm, such as about 0.5 to 10 µm, about 0.5 to about 5 µm, about 1 to about 5 µm, or about 2 to about 5 µm.
The powdered crystalline molecular sieve preferably has a D90 particle size of less than 10 µm. For example, the powdered crystalline molecular sieve may have a D90 particle size in the range 2 to 9 µm, preferably 3 to 8 µm. The term "D90 particle size" as used herein refers to particle size distribution. A value for D90 particle size corresponds to the particle size value below which 90%, by volume, of the total particles in a particular sample lie. The D90 particle size may be determined using a laser diffraction method (e.g. using a Malvern Mastersizer 2000).
If desired, prior to forming the slurry in step (a) of the method of the invention, the molecular sieve may undergo a particle size reduction treatment such as jet milling, wet milling or steam assisted jet-milling.
The components to be mixed together in step (a) of the method of the invention may include two or more crystalline molecular sieves in an H⁺ or NH₄ ⁺ form. Consequently, the resulting catalyst layer formed in step (c) may comprise two or more different types of metal-loaded molecular sieve.
As used herein "active metal precursor" refers to a metal species which is capable of supplying an extra framework metal to the crystalline molecular sieve. The term "extra-framework metal", as used herein, refers to a metal that resides within the molecular sieve (i.e. within the micropore structure, either in an ion-exchange position or non-ion exchange position) and/or on at least a portion of the molecular sieve surface (for example, in an ionic or oxide form), and does not include metal atoms of the tetrahedral units which constitute the framework of the molecular sieve. It will be understood that additional metal species may be present in the slurry formed in step (a) which would not themselves take part in metal-loading.
By "insoluble active metal precursor" it is meant an active metal precursor which is insoluble in water. In particular, the insoluble active metal precursor may have a water solubility of less than 1 g/100ml, for example less than 0.1g/100ml or less than 0.01g/100ml. The property water solubility is a measure of the quantity of a material that will dissolve in a certain volume of water at a specified temperature and pressure to form a saturated solution. As used herein, the term "water solubility" in relation to the insoluble active metal precursor, means the quantity (in grams) of the insoluble active metal precursor which will dissolve in 100 millilitres of water (g/100ml) at a temperature of 20°C and a pressure of 1 atmosphere.
Suitable insoluble active metal precursors include certain metal salts. In particular, the insoluble active metal precursor is a metal carbonate.
The insoluble active metal precursor preferably comprises a metal salt which undergoes thermal decomposition by thermolysis at temperatures of less than 500°C.
The insoluble active metal precursor comprises a salt of a transition metal. The insoluble active metal precursor comprises one or more insoluble salts of copper.
In particular, the insoluble active metal precursor is copper carbonate.
Specifically, the insoluble active metal precursor is copper (II) carbonate.
In addition to the insoluble active metal precursor, the components to be mixed in step (a) may further include a soluble (i.e. soluble in water) active metal precursor. Suitable soluble active metal precursors may include soluble metal salts, such as metal acetates or metal nitrates, or a mixture of any two or more thereof. In one example, the insoluble active metal precursor may comprise copper carbonate and the soluble active metal precursor may comprise cerium acetate.
The relative quantities of the molecular sieve and the insoluble active metal precursor employed in step (a) will depend on the targeted metal loading of the molecular sieve and the quantity of any soluble active metal precursors employed. Metal-loaded molecular sieve present in the catalyst layer produced in step (c) may have a metal-loading of ≥ 0.1% to ≤ 10 % by weight, preferably ≥ 0.1% and ≤ 7% by weight, more preferably ≥ 0.1% and ≤ 5% by weight.
In particular the crystalline molecular sieve being a zeolite, the relative quantities of the molecular sieve, the insoluble active metal precursor and any soluble active metal precursors employed in step (a) may be selected to provide a metal to alumina ratio in the metal-loaded zeolite in the range 0.2 to 0.5, preferably 0.3 to 0.5.
As used herein, the term "aqueous solvent" refers to an aqueous liquid medium (i.e. a water-containing liquid medium) and does not necessarily denote that any components are dissolved therein. For example, the aqueous solvent may be a water-containing liquid medium in which, during step (a), components (i) and (ii) become dispersed. However, it will be understood by the skilled person, that partial or full dissolution of some components in the aqueous solvent may occur. For example, rheology modifiers which may optionally be employed in step (a) may themselves dissolve in the aqueous solvent. The aqueous solvent is water. The water may be deionised or demineralised water.
The slurry formed in step (a) has a solids content of up to 50 wt%. By "solids content" it is meant the proportion of solid material present in the slurry based on the total weight of the slurry. The solids content of the slurry is preferably in the range 30 to 50 wt%, more preferably in the range 30 to 48wt%.
The components to be mixed together in step (a) may further include binder components, rheology modifiers and/or other additives.
In particular, the components to be mixed together in step (a) may further include a binder component selected from alumina, alumina precursors (such as boehmite and/or bayerite), aluminium hydroxide, TiO₂, SiO₂, ZrO₂, CeZrO₂, SnO₂, an aluminophosphate, non-zeolitic aluminosilicate, silica-alumina, clays or mixtures thereof.
The binder may be present in the slurry in an amount in the range 5 to 15 wt. %, preferably 8 to 12.5 wt.%, for example 10 to 12.5 wt.% based on total weight of the slurry.
The components to be mixed together in step (a) may further include a rheology modifier. The rheology modifier may be selected from a polysaccharide, a starch, a cellulose, an alginate, or mixtures thereof. The rheology modifier may be present in the slurry in an amount of up to 0.4 wt%, preferably ≤ 0.2 wt.%.
Optionally, the components to be mixed together in step (a) may further include organic additives, such as pore formers, surfactants, and/or dispersants as processing aids.
In some embodiments, the components to be mixed together in step (a) may further include additional catalytically active materials (such as materials active for catalysis of ammonia slip), for example, where it is desired that the catalyst article is multi-functional (i.e. performs more than catalytic function).
The relative quantities of each component employed in step (a) may be selected such that the slurry has the required solids content, and such that the catalyst layer formed in step (c), after removal of solvent and any organics, comprises the desired proportion of metal-loaded molecular sieve. This is well within the capabilities of the skilled person. Preferably, the relative quantities of each component employed in step (a) are chosen such that the catalyst layer formed in step (c) comprises 85 to 92 wt% of metal-loaded molecular sieve and 8 to 15 wt% binder.
In step (a) the slurry is formed by mixing together the components. Preferably, the slurry is substantially uniform (e.g. homogeneous), that is, the distribution of components throughout the slurry is substantially even. The components may be mixed by any suitable method. Preferably, the components are mixed by stirring.
Optionally, the pH of the slurry may be adjusted by the addition of an acid or a base. Advantageously, it has been found that variability of the pH of the slurry has little impact on the performance of SCR catalysts prepared according to the method of the invention. This is in contrast with some prior art methods where pH of the washcoat composition is known to influence the performance of the final catalyst.
Step (a) may be carried out at ambient temperature. Preferably, step (a) is carried out at a temperature in the range 10 to 30°C, preferably 18 to 28°C.
A particular advantage of the present invention is that the slurry formed in step (a) may be used directly as a washcoat composition. Thus, the slurry formed in step (a) may be employed directly in step (b) without any additional processing steps.
In step (b), the slurry formed in step (a) may be coated onto a substrate by washcoating techniques well known in the art. One such method involves positioning a monolith substrate such that the channels have a substantially vertical orientation, applying washcoat to a first face of the substrate (e.g. an upper face) and subjecting an opposite, second face (e.g. a lower face) of the substrate to at least a partial vacuum to achieve movement of the washcoat through the channels. The monolith substrate may be coated in a single dose wherein washcoat may be applied to the substrate in a single step with the substrate remaining in a single orientation. Alternatively, the substrate may be coated in two doses. For example, in a first dose the monolith substrate is in a first orientation with a first face uppermost and a second face is lowermost. A coating is applied to the first face and coats a portion of the length of the substrate. The substrate is then inverted so that the second face is uppermost. A coating is then applied to the second face in order to coat the portion of the substrate that was uncoated by the first dose. WO 99/47260 describes a general method for coating a monolithic substrate.
The coating should be applied to the substrate in an amount which is sufficient to provide the desired washcoat loading. Preferably, the coating is applied in an amount sufficient to provide a washcoat loading in the range 0.5 to 5 g/in³, preferably in the range 1.5 to 3.5 g/in³.
The substrate is preferably a honeycomb monolith substrate. Honeycomb monoliths are well known in the art. "Honeycomb monolith substrate" as defined herein includes metal and ceramic flow-through monoliths having a plurality of channels or cells which extend longitudinally along the length of the substrate structure and wherein the channels are open at both ends thereof; and metal and ceramic filters including ceramic wall-flow filters having a plurality of channels or cells which extend longitudinally along the length of the substrate structure and wherein channels at a first end of the substrate that are open are blocked at the opposite end and channels that are open at the opposite end are blocked at the first end, the arrangement being such that every other adjacent cell has an open end (or a blocked end) on the first end of the wall-flow filter and a blocked end (or an open end) on the opposite end thereof so that when an end of the wall-flow filter is viewed it resembles a chess board of open and blocked channels. Fluid communication between the open channels at the first end of the wall-flow filter and the open channels of the opposite end thereof is via the porous wall structure of the wall-flow filter.
Alternatively, the substrate may be a plate-type substrate.
The substrate may be an inert substrate. The substrate may be composed of a ceramic material or a metallic material. For example, the substrate may be made or composed of cordierite (SiO₂-Al₂O₃-MgO), silicon carbide (SiC), Fe-Cr-Al alloy, Ni-Cr-Al alloy, aluminum titanate or a stainless-steel alloy.
Where it is desirable that a catalyst article is multi-functional (i.e. it simultaneously performs more than catalytic function), the substrate may already possess catalytic activity prior to being coated with the slurry formed in step (a) of the first aspect. For example, the substrate may be an all-active extrudate. Alternatively, the substrate may already comprise a first washcoat layer. In this example, the slurry formed in step (a) may be coated as a second washcoat layer on top of the first, and/or, where the first washcoat layer does not cover the entire length of the substrate, may be coated as an adjacent or overlapping washcoat layer. For example, where the present invention provides an SCR catalyst, the slurry may be coated on top of or at a position which in use would be upstream of an ASC catalyst.
In principle, the substrate may be of any shape or size. However, the shape and size of the substrate is usually selected to optimise exposure of the catalytically active materials in the catalyst article to the exhaust gas in use.
Step (b) may be carried out at ambient temperature. Preferably, step (b) is carried out at a temperature in the range 10 to 35°C, preferably in the range 10 to 30°C, more preferably 18 to 28°C.
Most preferably steps (a) and (b) are both carried out at a temperature in the range 10 to 35°C, for example in the range 10 to 30°C or 18 to 28°C.
The coated substrate formed in step (b) may undergo a drying process prior to calcination in step (c). Thus, the method of the invention may further comprise drying the coated substrate formed in step (b) prior to carrying out step (c).
Drying of the coated substrate may be carried out at temperatures of less than 120°C. For example, drying of the coated substrate may be carried out at a temperature of about 100°C. Drying may be carried out statically (for example, using a batch oven) or continuously (for example, using a belt furnace).
In step (c) of the method of the invention, the (optionally dried) coated substrate formed in step (b) undergoes calcination to form a catalyst layer on the substrate comprising metal-loaded molecular sieve. The term "calcine" or "calcination" refers to a thermal treatment step. Calcination causes the catalytically active material to become fixed to the substrate as well as removal of any remaining solvent and any residual organic components, such as organics derived from decomposition of active metal precursors or from organic additives included in the slurry formed in step (a).
Without wishing to be bound by theory, it is believed that at least some metal-loading of the molecular sieve occurs during calcination of the coated substrate. For example, it is possible that a solid-state ion-exchange takes place during calcination.
Calcination of the coated substrate may be carried out via techniques well known in the art. In particular, calcination may be carried out statically (for example, using a batch oven) or continuously (for example, using a belt furnace).
Preferably, calcination step (c) is carried out at temperatures up to 550°C, preferably in the range 450 to 550°C.
Preferably, the coated substrate is calcined for up to 3 hours, preferably from 30 minutes to 2 hours.
The calcination carried out in step (c) may comprise multiple thermal treatment steps, for example, the coated substrate may be subjected to a first thermal treatment at a first temperature, and then subjected to a second thermal treatment at a second temperature.
Drying and calcination may optionally be combined in a continuous process, wherein the coated substrate is conveyed on a belt furnace through multiple heating zones, each zone being set to a different temperature.
The catalyst article prepared according to the method of the invention may be employed for treating a flow of a combustion exhaust gas. That is, the catalyst article can be used to treat an exhaust gas derived from a combustion process, such as from an internal combustion engine (whether mobile or stationary), a gas turbine or a power plant (such as a coal or oil-fired power plant). A preferred application for the catalyst article is in an exhaust system for an automotive vehicle. In particular, the catalyst article may be employed as an SCR catalyst.
In some embodiments, for example, where it is desired that the catalyst article is multi-functional (i.e. it simultaneously performs more than catalytic function), subsequent to step (b), the method of the invention may include a step of coating a further washcoat composition onto the substrate. The further washcoat composition may be applied directly on top of the coating applied in step (b) or at a position proximal thereto. Such a step may be carried out either before or after step (c).
The catalyst article may be part of an emission gas treatment system wherein the catalyst article is disposed downstream of a source of a nitrogenous reductant.

Examples

The invention will now be further described with reference to the following examples, which are illustrative, but not limiting of the invention, which is defined by the appended claims.

• Comparative Example

Particulate SSZ-13 (CHA) zeolite was combined with water to form a slurry having a solids content of 37wt% and then the particle size of the zeolite was reduced to a D90 of 7µm using a bead mill.
Copper acetate was added to the slurry and the resulting mixture was heated to 70°C for 4 hours. After cooling to room temperature, a sample of the slurry was taken for ICP analysis which confirmed that greater than 84% of copper uptake had taken place.
To replace water lost by evaporation during heating, additional water was added to the slurry such that the solids content was adjusted back to 37wt%. Tetraethylammonium hydroxide (TEAOH) was then added to complex any free copper ions remaining in the supernatant.
A binder component (water soluble boehmite - Dispersal, available from Sasol) was then added to the slurry, which was then stirred under continuous high shear conditions until it was homogenized.
In order that the slurry was suitable for washcoating, the rheology of the slurry was adjusted by the addition of a cellulose rheology modifier. The pH of the slurry was adjusted to 3.8 by the addition of a base.
The final slurry was then washcoated onto a square-cell, ceramic flow-through substrate using a vacuum deposition washcoating technique (as described in WO 99/47260 ). The coated substrate was then dried to complete dryness using a dynamic line drier. The dried coated substrate was then calcined on a dynamic line calciner at 500°C for at least 30 minutes to form a catalyst layer on the substrate.
The quantitative proportions of the starting materials were selected such that catalyst layer contained 87.5 wt% copper-loaded zeolite and 12.5 wt% alumina.

• Example 1

Particulate SSZ-13 (CHA) zeolite was combined with water to form a slurry having a solids content of 37% and the particle size of the zeolite was reduced to a D90 of 7µm using a bead mill.
Copper carbonate was added to the slurry and the resulting mixture was stirred under high sheer conditions for a minimum of 20 minutes. The quantity of copper carbonate was selected to give an equivalent wt% of copper in the final slurry as provided in the Comparative Example.
A binder component (water soluble boehmite - Dispersal, available from Sasol) was then added to the slurry, which was then stirred under continuous high shear conditions until it was homogenized.
In order that the slurry was suitable for washcoating, the rheology of the slurry was adjusted by the addition of a cellulose rheology modifier. The pH of the slurry was adjusted to approximately 3.8 by the addition of a base.
The final slurry was then washcoated onto a substrate having the same shape and dimensions as that employed in the Comparative Example in the same manner as described in relation to the Comparative Example. The coated substrate was then dried and calcined in the same manner as described in the Comparative Example.
The quantitative proportions of the starting materials were selected such that final catalyst layer contained 87.5 wt.% copper and zeolite and 12.5 wt.% alumina.

• Example 2

Example 1 was repeated except that the pH of the slurry was adjusted to 4.

• Example 3

Example 1 was repeated except that the pH of the slurry was adjusted to 7.

• Catalyst testing

Identical volume core samples were taken from the catalyst articles prepared in each of the Comparative Example and Examples 1 to 3 and tested in a synthetic catalytic activity test (SCAT) apparatus using the following inlet gas mixture at selected inlet temperatures: 500ppm NO, 750ppm NH₃, 10% H₂O, 5% O₂, 350ppm CO, balance N₂ at a flow rate of 31.6 L/min. The catalyst samples were tested both in a fresh condition and after hydrothermal ageing (800°C for 16 hours in 10% water).
The results are shown in Figures 1 and 2.
Figure 1 compares the NOx conversion rate and N₂O selectivity achieved by the catalyst articles of Example 1 and the Comparative Example at the selected inlet temperatures.
Figure 2 compares the NOx conversion rate and N₂O selectivity achieved by the catalyst articles of Examples 1 to 3 at the selected inlet temperatures.
As demonstrated by the data shown in Figure 1, the catalyst article of Example 1 achieves comparable NOx conversion rates and similar or slightly improved N₂O selectivity compared to the Comparative Example.
Advantageously, the preparation of Example 1 required fewer process steps and reduced water and energy consumption compared to the overall preparation of the Comparative Example.
As demonstrated by the data shown in Figure 2, changes in pH of the slurry for washcoating has little impact on the performance of final catalyst articles.

Claims

A method for forming a catalyst article comprising:
(a) forming a slurry by mixing together at least the following components:
(i) a crystalline molecular sieve in an H⁺ or NH₄ ⁺ form, wherein the crystalline molecular sieve is a small pore zeolite having a Framework Type that is CHA;

(ii) a water-insoluble active metal precursor, wherein the water-insoluble active metal precursor is copper (II) carbonate;

(iii) an aqueous solvent, wherein the aqueous solvent is water;
wherein the slurry has a solids content of up to 50 weight% and wherein step (a) is carried out at a temperature in the range 10 to 35°C;

(b) coating a substrate with the slurry formed in step (a); and

(c) calcining the coated substrate formed in step (b) to form a catalyst layer on the substrate.
A method as claimed in claim 1 wherein in step (a) the components to be mixed together further include (iv) a binder component and/or (v) a rheology modifier.
A method as claimed in claim 2 wherein the binder component is selected from alumina, alumina precursors, aluminium hydroxide, TiO₂, SiO₂, ZrO₂, CeZrO₂, SnO₂, aluminophosphate, non-zeolitic aluminosilicate, silica-alumina, clays or mixtures thereof.
A method as claimed in claim 2 or 3 wherein the rheology modifier is selected from a polysaccharide, a starch, a cellulose, an alginate, or mixtures thereof.
A method as claimed in any preceding claim wherein the slurry formed in step (a) has a solids content in the range 30 to 50 wt%, or in the range 30 to 48 wt%.
A method as claimed in any preceding claim wherein step (a) is carried out at a temperature in the range 10 to 30°C, preferably in the range 18 to 28°C.
A method as claimed in any preceding claim wherein step (b) is carried out at a temperature in the range 10 to 35°C, preferably in the range 10 to 30°C, more preferably in the range 18 to 28°C.